1. Field of the Invention
The present invention relates to a device for digitally coating textile. In particular, it relates to a device for coating a textile using a continuous flow inkjet technique to provide accurate coating characteristics. It furthermore relates to a method of coating textiles using such a technique and to the textile produced thereby.
2. Description of the Related Art
Coating is one of the operations frequently performed during the production of textiles. Roughly five stages can be distinguished in such production; the fibre production; spinning of the fibres; the manufacture of cloth (for instance woven or knitted fabrics, tufted material or felt and non-woven materials); the upgrading of the cloth; and the production or manufacture of end products. Textile upgrading covers a number of operations such as preparing, bleaching, optically whitening, colouring (painting and/or printing), coating and finishing. These operations generally have the purpose of giving the textile the appearance and physical characteristics that are desired by the user. Coating of the textile is one of the more important techniques of upgrading and may be used to impart various specific characteristics to the resulting product. It may be used for making the substrate fireproof or flameproof, water-repellent and/or oil repellent, non-creasing, shrink-proof, rot-proof, non-sliding, fold-retaining and/or antistatic.
Conventional processes for upgrading textile are composed of (FIG. 1) a number of part-processes or upgrading steps, i.e. pre-treating the textile article (also referred to as the substrate), painting the substrate, coating the substrate, finishing the substrate and the post-treatment of the substrate. The usual techniques for applying a coating on solvent or water basis are the so-called knife-over-roller, the dip and the reverse roller coaters. A dispersion of a polymer substance in water is usually applied to the cloth and excess coating is then scraped off with a doctor knife. Certain characteristics are difficult to achieve using such conventional coating techniques and must be attained by other techniques. In order to provide a full colour to the article, painting may take place by immersing the textile article in a paint bath, whereby the textile is provided on both sides with a coloured substance. For other effects, foularding (impregnating and pressing) may be used.
Each of the upgrading steps shown in FIG. 1 consists of a number of operations. Different treatments with different types of chemicals are required, depending on the nature of the substrate and desired end result. For the upgrading steps of printing, painting, coating and finishing four recurring steps can generally be distinguished which often take place in the same sequence. These treatments are referred to in the professional field as unit operations. These are the treatments of impregnation (i.e. application or introduction of chemicals), reaction/fixing (i.e. binding chemicals to the substrate), washing (i.e. removing excess chemicals and auxiliary chemicals) and drying. These unit operations may also need to be repeated a number of times for each upgrading step e.g. repeated washing cycles. Large quantities of chemical reagents and water are generally used which entails a relatively high environmental impact, a long throughput time and relatively high production costs.
It is moreover usual at present to carry out the different upgrading steps of the textile in separate devices. This means that for instance the painting is performed in a number of paint baths specially suited for the purpose, the printing and coating are carried out in separate printing devices and coating machines, while finishing is carried out by yet another device. Because the different operations are carried out individually in separate devices, the treating of the textile requires a relatively large area, usually spread over different room areas.
It is thus desirable to provide methods of upgrading, i.e. painting, coating and finishing, a substrate of textile where the above stated drawbacks and other drawbacks associated with conventional processes are reduced.
Various attempts have been made to use inkjet printing techniques for performing upgrading steps. In particular, inkjet printers have been suggested for printing an image onto a textile. Conventional inkjet techniques known for printing onto paper media have however been found difficult to implement for textile production where textile widths of more than 1 meter are standard and production speeds of 20 meters per minute or more are required in order for the process to be efficient. In particular, conventional inkjet printers comprise a printing head that moves backwards and forwards across the medium. The printing head has a number of nozzles through which streams of ink droplets may be fired. These print heads operate according to the dot-on-demand principle i.e. they are electronically controlled to deposit an ink droplet or not according to the image to be printed. The medium is fed forwards intermittently after each pass of the printing head. Both the intermittent feed and the drop-on-demand control cause the process to be too slow for practical use. Feed velocities of 2 meters per minute are currently achievable using such methods for textile printing. A process is known from U.S. Pat. No. 4,702,742 in which a conventional printing device is used to print onto white cloth sheets. A further process is suggested in German patent application No. DE 199 30 866 in which both ink and a fixing solution are applied to a textile using a conventional inkjet head.
In particular, it has been found that conventional inkjet printing devices are unsuitable for the purpose of coating textiles. This is particularly the case when used on fibrous textiles in which gaps exists between the adjacent fibres, especially for coarsely woven or knitted textiles. Typical nozzle diameters used in conventional inkjet devices are relatively small in order to provide fine pixel definition. It has been found that the droplets produced by such nozzles tend to pass into or even through the gaps providing a less than adequate surface finish. It has also been found that despite the advantages of printing onto textile using inkjet techniques, pixel definition of images produced on coarse textiles is often deficient due to the coarseness of the fibre structure and other effects such as wicking which may not be homogenous in all directions.